The continuous measurement of pressure at strategic locations in a gun tube during the propellant ignition and burning process produces pressure-time curves which are used to evaluate system performance and safety. During the past several years the accuracy and reliability of these pressure measurements have become increasingly more critical due to the development of high performance weapon systems requiring more stringent design tolerances and optimum safety limit determinations. The pressure transducer is normally the major source of error in any ballistics pressure measurement system because of the adverse environment in which the transducer has to perform. One of the major problems encountered is shielding the pressure sensing element of the transducer from heat and residue generated by the burning propellant. There are a variety of methods used to provide this shielding on conventional transducers and the tendency is to over protect which also affects the frequency response and accuracy of the transducer. In some cases a thin layer of ceramic, asbestos, mica and/or modeling clay is used in conjunction with a high temperature grease. In order to maintain the required quality of data, the conventional transducers have to be removed from the weapon after every one to ten rounds (depending on the design of the transducer and firing conditions) for cleaning, recalibrating and replacing the shielding materials. This is a time consuming operation and frequently interferes with the firing schedule. The only thermal protection required on this new pressure transducer is a high temperature, general purpose chassis grease which can be quickly applied through a specially designed needle valve without removing the transducer from the weapon. This method of applying grease also flushes out propellant residue.
The design of this new transducer also allows the pressure sensing element to be electrically insulated from the weapon which eliminates ground loops and improves the signal-to-noise ratio. Differences in ground potential between the weapon and instrumentation chassis ground produce ground loops which appears as noise on the pressure-time signal. This condition can be very difficult to correct with conventional pressure transducers and normally causes delays in testing or loss of data if the noise is initiated by the firing pulse on electrically fired weapons.
Another advantage of this invention is that it utilizes tourmaline crystals [(Na, Ca) (Li, Mg, Fe, Al) (Al, Fe).sub.6 B.sub.6 Si.sub.6 O.sub.27 (O, OH, F).sub.4 ], which have approximately ten times the output of the more common quartz piezoelectric pressure transducers. This increases the measurement resolution and further improves the signal-to-noise ratio. The tourmaline crystal is also hydrostatically sensitive to pressure; therefore, no piston-cylinder assembly or flush diaphragm is required to transfer the force from the pressure source to the crystal. This feature eliminates what is referred to as "torque sensitivity" because the crystal assembly is completely surrounded by the pressure source which prevents stresses from being transferred to the crystal assemby due to the mounting torque. The primary objections to the use of tourmaline pressure transducers in the past have been their susceptibility to damage during firing, excessive maintenance/calibration requirements and difficulty in miniaturizing the crystal assembly for use in the smaller weapons such as the 20 MM. These problems have been solved with this new design.